The invention relates to a method for motor reduction while closing the lock-up clutch for an automatic transmission of a motor vehicle.
According to the prior art an automatic transmission contains a hydrodynamic converter with a lock-up bridge clutch, at least one planetary gear set and two shiftable frictional-engagement connections, in the form of a friction clutch and transmission brake, which serve to transmit the torque.
By virtue of hydrodynamic torque transmission in the converter, the rotational speeds of the input (e.g. motor) and turbine can assume different values. According to the prior art, another function of the converter is to attenuate awkwardnesses of the motor.
A motor reduction, according to the prior art, is introduced while closing the lock-up bridge clutch. The motor power is reduced to a fixed value.
This procedure results in an engagement jerk when employing the disks of the lock-up bridge clutch. In addition, at the end of the slipping phase of the lock-up bridge clutch, an engagement jerk is also produced which causes a ringing of the drive line.
One other undesired effect is a clear torque excess on the output during the whole operation, i.e. up to the end of the slipping phase.
Therefore, the problem on which this invention is based is to outline, for motor reduction while closing the lock-up bridge clutch, a method which overcomes the undesired effects of the former procedure. The disc employment operation in particular must develop without engagement jerk and a torque excess on the output must be prevented.
Accordingly, it is proposed to bring up the starting value of the motor reduction via an adjustable ramp in the period of time when the disks of the lock-up clutch are being employed.
In addition, the motor reduction can be adjusted with reference to a characteristic field. A preferred manner is to store in the characteristic field the amount of the motor reduction via the measured rotational speed ratio n_ue with n_ue=n_turbine/n_pump. The controller principle corresponds to a characteristic field controller whereby an optimized motor reduction is ensured.
The motor reduction automatically terminates by measuring the variable n_ue so that at the end of the slipping phase the torque connection is optimized. For n_ue=1, the motor reduction is terminated.
With the solution proposed, the gear shift is optimized so that no engagement jerk is produced. This is obtained by the fact that the torque excess on the output is to a great extent compensated by the motor reduction.
In addition, the load of the disks of the lock-up bridge clutch is reduced by the motor reduction without negative effect upon the output torque.